FIELD OF THE INVENTION
Although many active control laws have been developed for vibration suppression in structural systems, few of these meet the practical requirements of large force capability, robustness, reliability, and simplicity. Recent advances in control have resulted in a great many high performance control design methods. Most, however, require actuators capable of producing large forces, with high degrees of reliability and fidelity to their modeled behavior. While a great deal of research is underway to address such issues systematically, most efforts seem to be aimed at developing more complex hardware and control law designs.
Recently, switching type control laws have been implemented in experimental structures and in operational buildings. See, J. N. Yang, Z. Li, and J. C. Wu, "Control of Seismic Excited Buildings Using Active Vatiable Stiffness System," Proceedings of the 1994 American Control Conferences, Baltimore, Md., pp 1083-1088; G. Leitmann and E. Reithmeirer, "Semiactive Control of a Vibrating System by Means if Electrorzeological Fluids"; Dynamics and Control, vol 3, no 1, January 1993, pp 7-34; N. H. McClamroch, H. P. Gavin, D. S. Ortiz, and R. D. Hanson, "Electrorheological Dampers and Semi-Active Structural Control", Proceedings of 1994 CDC, pp 93-103; and W. N. Patten and R. L. Sack, "Semiactive Control of Civil Engineeting Structures", Proceedings of 1994 ACC, Baltimore, Md., pp 1078-1082. These prior art teachings are among the examples of alternative approaches that have been receiving increasing attention. Sliding mode techniques have been used to design control schemes for a building equipped with bracing that can be locked or unlocked, hence changing the effective stiffness of the building. See Yang, supra. These active variable stiffness (AVS) systems have been installed on full scale buildings and shown to be effective for control of structures under strong seismic excitations. See, T. T. Soong, Active Structural Control: Theory and Application, Longman Scientific, New York, 1990; and T. Kobori, S. Kamagata, "Dynamic Intelligent Buildings: Active Seismic Response Control," in Intelligent Structures: Monitoring and Control, Elsevier Applied Science, N.Y. at 279-82 (1991).
In addition to these techniques, general Lyapunov stability methods have been applied to a class of parametric control problems (i.e., systems in which important parameters of the system are altered to obtain desirable behavior). See, Leitmann, supra. In particular, systems where the effective damping and stiffness can be changed continuously, within a limited range, through the use of electro-rheological fluids.
Another application of the use of novel materials and the associated modeling and design for structural control has recently been developed as described by McClamroch, supra. These methods are often called semi-active due to the simple fact the amount of power and force needed are quite minimal (e.g., enough to control the orifice in a valve or the current through the fluid).
Another example of such devices has been developed by Patten, supra, wherein hydraulics are used to reduce the strain energy of structures.
Seismic hydraulic cylinders used in systems to change and damp the resonant frequencies of buildings are described in Kobori et.al., "High Damping Device for Seismic Response Controlled Structure," U.S. Pat. No. 5,347,771 (1994); Kobori, et. al., "Variable Damping and Stiffness Structure," U.S. Pat. No. 5,036,633 (1991); Ishii et. al, "Compound Seismic Response and Wind Control System," U.S. Pat. No. 5,025,599 (1991); Kobori et. al., "Active Seismic Response Control System for Use in Structure," U.S. Pat. No. 5,065,552 (1991); Kobori et. al., "Variable Damping Device for Seismic Response Controlled Structure," U.S. Pat. No. 5,311,709 (1994); Dickinson Jr. et. al, "Fluid Flow Control Device", U.S. Pat. No. 5,561,574 (1971); Kobori et. al., "Apparatus for Accelerating Response Time of Active Mass Damper Earthquake Attenuator," U.S. Pat. No. 5,022,201 (1991); Kobori et. al., "Cylinder Lock Device for Use in Structure," U.S. Pat. No. 5,147,018 (1992); Isshi et. al., "Safety Monitoring Method for Use in Active Seismic Response and Wind Control System," U.S. Pat. No. 5,193,323 (1993); Isshi et. al., "Safety Monitoring Device for Use in Active Seismic Response and Wind Control System," U.S. Pat. No. 5,046,290 (1991); Sjoestroem, "Suspension System with Improved Response Damping and a Method for Regulating the Suspension System," U.S. Pat. No. 5,337,864 (1994); and Nemir, "Method and Apparatus for Damping Vibrations," U.S. Pat. No. 5,168,673 (1992). Although these devices use hydraulic cylinders with controlled valves, their method of control is generally directed to controlling or damping frequencies of vibration and little direct attention is paid to optimizing or tracking the absorption of energy in the system.
What is needed is an apparatus and method for its control that requires minimal power, e.g. only enough to open and close a valve, and which will act only to reduce the total amount of energy of the system. The system should ideally be decentralized in the sense that the behavior of each actuator is dependent only on the degrees of freedom it is directly attached to. It should be reliable, in that it does not require full state sensing (or an observer), and the associated control law must be simple with minimal hardware complexity. Furthermore, the control law should not require an exact model of the system it damps and, hence, must be robust with respect to parameter uncertainty.